The present invention relates to thermoluminescent alpha particle responsive dosimeters and apparatus which are useful for locating uranium deposits.
A thermoluminescent dosimeter is comprised of a phosphor material which after exposure to radiation such as alpha particles, will give off light or luminesce when heated to a characteristic temperature. The radiation causes some of the electrons of the phosphor material to be raised to an excited state where they remain trapped for an extended time unless the phosphor is heated to a characteristic temperature. Upon heating, the excited state electrons return to the ground state, giving off a pulse of light at a characteristic wavelength. The phosphor material may be formed as a compacted body or mixed and compacted with an inert carrier such as polytetrafluorethylene, as seen in U.S. Pat. No. 3,471,699.
A variety of phosphor materials are known which exhibit thermoluminescence, such as activated lithium and calcium fluorides, and calcium sulfate activated by dysprosium.
When a thermoluminescent dosimeter is made of a particular sensitivity by controlling its thickness, it can be used as taught by U.S. Pat. No. 4,053,772 in locating underground uranium deposits. The thermoluminescent dosimeter described in the aforementioned patent comprised a disk or body of phosphor and polytetrafluoroethylene which is "Teflon" a DuPont trademarked material. The dosimeter disk or body had a thickness of from about 5 to 18 mg/cm.sup.2, and a thin aluminum layer surrounded the dosimeter disk. The aluminum layer was about 0.030 mil thick or about 0.8 micrometers to permit passage of alpha, beta and gamma radiation, and to act as a protective layer excluding moisture and dust. The dosimeter was disposed in an inverted cup placed in a test hole, so that alpha emissive radon gas (Radon-222), which percolates up through the ground activated the phosphor of the dosimeter. The dosimeter also detects alpha particles emitted by radon daughters. The dosimeters are collected and heated to permit a reading of the emitted light which is a function of the radon gas encountered.
The dosimeter was a very thin disk of about 1 to 3 mils thickness sandwiched between very thin 0.03 mil thick aluminum foil. This very thin aluminum foil is difficult to work with during manufacture and can be easily damaged during field use resulting in moisture or soil exposure to the dosimeter disk and degraded sensitivity. The thin aluminum foil was also used to prevent transmission of light and particularly the ultraviolet component from reaching the dosimeter, since ultraviolet radiation can cause the excited electrons to return to the ground state.